22

Hoveyda–Grubbs type metathesis catalystimmobilized on mesoporous molecular

sieves MCM-41 and SBA-15Hynek Balcar*, Tushar Shinde, Naděžda Žilková and Zdeněk Bastl

Full Research Paper Open Access

Address:J. Heyrovský Institute of Physical Chemistry of AS CR, v.v.i,Dolejškova 3, 182 23 Prague 8, Czech Republic

Email:Hynek Balcar* - balcar@jh-inst.cas.cz

* Corresponding author

Keywords:alkene metathesis; catalyst immobilization; hybrid catalysts;mesoporous molecular sieves; Ru–alkylidene complexes

Beilstein J. Org. Chem. 2011, 7, 22–28.doi:10.3762/bjoc.7.4

Received: 31 August 2010Accepted: 16 November 2010Published: 06 January 2011

Guest Editor: K. Grela

© 2011 Balcar et al; licensee Beilstein-Institut.License and terms: see end of document.

AbstractA commercially available Hoveyda–Grubbs type catalyst (RC303 Zhannan Pharma) was immobilized on mesoporous molecular

sieves MCM-41 and on SBA-15 by direct interaction with the sieve wall surface. The immobilized catalysts exhibited high activity

and nearly 100% selectivity in several types of alkene metathesis reactions. Ru leaching was found to depend on the substrate and

solvent used (the lowest leaching was found for ring-closing metathesis of 1,7-octadiene in cyclohexane – 0.04% of catalyst Ru

content). Results of XPS, UV–vis and NMR spectroscopy showed that at least 76% of the Ru content was bound to the support

surface non-covalently and could be removed from the catalyst by washing with THF.

22

IntroductionRu–alkylidene complexes (Grubbs and Hoveyda–Grubbs cata-

lysts, 1 and 2, respectively, Figure 1) belong to the most active

and frequently used metathesis catalysts. These catalysts are

important tools in organic synthesis due to their high tolerance

of heteroatoms in substrate molecules. Immobilization of these

complexes on solid supports has attracted attention, because it

opens the possibility for easy catalyst–product separation

and catalyst reuse. The production of products free from

catalyst residues is especially important in the synthesis of

drugs and some other fine chemicals. Several strategies have

been developed for the immobilization of Grubbs and

Hoveyda–Grubbs catalysts on solid supports [1-6]. Generally,

the complex can be attached to the supports: (a) by exchanging

halide ligands X [7-11], (b) by exchanging phosphine and NHC

ligands L [12,13], and (c) through the alkylidene ligand [14-19].

For these purposes, special ligand molecules with tags suitable

Beilstein J. Org. Chem. 2011, 7, 22–28.

23

Figure 2: Zhan catalyst-1B.

for the reaction with the support surface (linkers) are used. This

usually requires a sophisticated synthetic procedure. Moreover,

the changes in the Ru coordination sphere may lead to the

decrease in catalyst activity (e.g., the exchange of chloro

ligands for carboxylates [10,11]).

Figure 1: Grubbs 1 and Hoveyda–Grubbs 2 catalysts.

Recently, a convenient method for the immobilization of

Hoveyda–Grubbs catalysts was reported [20]. A second genera-

tion Hoveyda–Grubbs catalyst was immobilized on silica

without any linkers by simply placing the Ru complex in

contact with silica in a suspension. Heterogeneous catalysts

(loading from 0.05 to 0.6 wt % Ru) were prepared, which were

active in ring-opening metathesis polymerization (ROMP) of

cyclooctene, ring-closing metathesis (RCM) of diallylsilanes

and several types of cross-metathesis reactions. High stability of

catalysts, reusability and low Ru leaching were also reported.

However, the mode of attachment of the Ru species on the silica

surface was unclear.

The aims of this paper are the following: to report the

immobilization of the Hoveyda–Grubbs type catalyst 3

(Figure 2, Zhan catalyst-1B) on mesoporous molecular sieves

SBA-15 and MCM-41 as supports with this simple immobiliz-

ation method; to describe the activity and stability of hetero-

geneous catalysts prepared; and to clarify the nature of the bond

between the Ru complex and the support surface. Mesoporous

molecular sieves are advanced siliceous materials [21], with

high surface area, high pore volume and narrow distribution of

pore size. Because of these unique qualities, they often find

applications as supports of modern catalysts, including those

used for metathesis reactions [22].

Results and DiscussionCatalyst activityMixing a toluene solution of 3 with dried MCM-41 and SBA-

15, respectively, for 30 min at room temperature led to green

solids, which after isolation and drying afforded heterogeneous

catalysts 3/MCM-41 and 3/SBA-15. The immobilization

proceeded almost quantitatively: 97% and 94% of the Ru com-

plex was transferred from the solution onto the MCM-41 and

SBA-15 surface, respectively. The established catalyst loading

was 0.98 wt % and 0.93 wt %, for 3/MCM-41 and 3/SBA-15,

respectively.

The catalytic activity was tested in the RCM of 1,7-octadiene

(Scheme 1, entry 1) and diethyl diallylmalonate (DEDAM)

(Scheme 1, entry 2), in the metathesis of methyl oleate

(Scheme 1, entry 3) and methyl 10-undecenoate (Scheme 1,

entry 4), and in the ROMP of cyclooctene (Scheme 1, entry 5).

Figure 3 shows conversion curves for the RCM of DEDAM in

dichloromethane, benzene, and cyclohexane, and the RCM of

1,7-octadiene in cyclohexane. For the RCM of DEDAM, 3 (as a

homogeneous catalyst) and 3/SBA-15 were used. The reaction

proceeded very rapidly in all solvents used (TOF at 10 min was

approximately 2500 h−1). No decrease in the reaction rate was

observed as a result of the immobilization of complex 3. For the

RCM of 1,7-octadiene, 3/SBA-15 and 3/MCM-41 in cyclo-

hexane were used. The reaction profile was the same for both

catalysts and the reaction rate was significantly lower compared

to the RCM of DEDAM. The selectivity was near 100% in all

cases (no side products were observed by GC).

The Ru leaching (in % of starting content of Ru in catalyst) for

the reactions shown in Figure 3 is given in Table 1. It was

observed that leaching was dependent on both the solvent and

substrate used in the reaction – the highest leaching was found

for the RCM of DEDAM in CH2Cl2 whilst negligible leaching

was observed for the RCM of 1,7-octadiene in cyclohexane.

Filtration experiments carried out with 3/SBA-15 (Figure 4)

showed that the catalytic activity was completely bound to the

solid phase for the RCM of 1,7-octadiene in cyclohexane. At

5 min after starting the reaction, about ½ of the volume of the

liquid phase was separated by filtration and transferred into a

new reactor, kept under identical reaction conditions. No

increase in conversion was observed in this reactor. For the

RCM of DEDAM in benzene, however, a considerable increase

Beilstein J. Org. Chem. 2011, 7, 22–28.

24

Scheme 1: Metathesis reactions studied.

Figure 3: Conversion curves for the RCM of DEDAM with 3 in CH2Cl2(open squares), 3/SBA-15 in CH2Cl2 (inverted filled triangle), 3/SBA-15in benzene (filled diamond), 3/SBA-15 in cyclohexane (filled triangle),and for the RCM of 1,7-octadiene with 3/SBA-15 (filled squares) and 3/MCM-41 (filled circles) in cyclohexane (T = 30 °C, molar ratio sub-strate/Ru = 600, c0

substrate = 0.2 mol/L).

of substrate conversion in liquid phase after its separation from

solid catalyst indicated that the Ru species released from the

solid catalyst were capable of initiating catalytic reactions.

Figure 5 shows the activity of 3/MCM-41 and 3/SBA-15 in the

metathesis of methyl oleate and methyl 10-undecenoate. The

reaction proceeded more slowly than in the case of the RCM of

DEDAM (TOF at 30 min = 260 h−1 for methyl oleate and

Table 1: Ru leaching for 3/SBA-15.

Reaction Solvent Ruleaching

Maximal Rucontent in producta

(1) cyclohexane 0.04% 0.6 ppm(2) benzene 4% 28 ppm(2) cyclohexane 9% 66 ppm(2) dichloromethane 14% 100 ppm

molar ratio substrate/Ru = 600, c0substrate = 0.2 mmol/mL, T = 30 °C

acalculated as the amount of Ru per weight unit of substrate

35 h−1 for methyl 10-undecenoate, both with 3/MCM-41). The

metathesis of methyl oleate reached the equilibrium after 2 h. In

the case of methyl 10-undecenoate, the reaction rate was lower

than for methyl oleate (because of probable non-productive

metathesis [23]) and the final conversion was about 65% (due to

the evolution of ethylene into the gas phase). There was no

significant difference observed in the conversion curves for

reactions with 3/MCM-41 and 3/SBA-15. Selectivity near 100%

was achieved in all cases.

The ROMP reactions were carried out for cyclooctene with both

3/MCM-41 and 3/SBA-15 (cyclooctene/Ru molar ratio = 500,

c0cyclooctene = 0.6 mol/L, T = 30 °C). High molecular weight

polymers (Mw = 250 000, Mn = 110 000) were obtained in high

yields (70% and 64% for 3/MCM-41 and 3/SBA-15, respective-

ly) after 3 h.

Beilstein J. Org. Chem. 2011, 7, 22–28.

25

Figure 4: Filtration experiments with 3/SBA-15. RCM of DEDAM inbenzene (circles), 1,7-octadiene in cyclohexane (squares), liquidphase in contact with solid catalyst (filled symbols), liquid phase afterfiltration (open symbols), arrows indicate time of filtration. Substrate/Rumolar ratio 600, T = 30 °C, c0

DEDAM = 0.22 mol/L, c01,7-octadiene = 0.16

mol/L.

Figure 5: Metathesis of methyl oleate (open symbols) and methyl10-undecenoate (filled symbols) with 3/MCM-41 (circles) and 3/SBA-15 (squares). Molar ratio substrate /Ru = 500, T = 30 °C, c0

substrate =0.15 mol/L.

The catalysts 3/MCM-41 and 3/SBA-15 exhibited some

features similar to those of Hoveyda–Grubbs catalyst

immobilized on silica [20]: (a) Filtration experiments proved

complete catalyst heterogeneity only for nonpolar solvents

(cyclohexane and hexane, respectively); (b) catalyst leaching in

these solvents was extremely low, ensuring more than one order

of magnitude lower Ru concentration in products compared to

the upper limit permissible in pharmaceutical production

(10 ppm [3]); and (c) similar catalyst activity was observed.

Although the slightly different reaction conditions applied in

[20] do not allow an exact comparison, the initial TOF values

achieved in [20] are of the same order as the TOF values

obtained in our work. However, in the case of the ROMP of

cyclooctene, catalysts 3/MCM-41 and 3/SBA-15 led to high

molecular weight polymers, whereas in [20] the formation of

polymer was not referred.

Interaction between the Ru complex and thesupportFor Hoveyda–Grubbs catalyst immobilized on silica, the

authors in [20] proposed a direct chemical interaction between

the Ru species and the silanol groups of the surface, instead of a

weak physisorption. In order to shed light on the mode of com-

plex attachment to the sieve surface, UV–vis and XPS spectra

were measured. In Figure 6, the UV–vis spectra of 3/SBA-15

and 3 in CH2Cl2 are compared. The bands at λ = 375 nm and at

λ = 600 nm (2 orders of magnitude lower ε, not visible in

Figure 6) in the spectrum of 3 reflect the d–d transition of the

Ru(II) atoms [24]. Supported catalyst 3/SBA-15 exhibits the

same spectrum suggesting no changes in the coordination

sphere of Ru atoms occurred during immobilization of 3 on the

sieve.

Figure 6: UV–vis spectra of 3/SBA-15 (curve 2) and of 3 (curve 1) indichloromethane (c = 0.001 mol/L, l = 0.2 cm).

Assuming non-covalent interactions between the Ru species and

the support surface, we attempted to wash out the Ru species

from 3/SBA-15 with THF-d8 and characterise the eluate by

NMR spectroscopy. About 100 mg of 3/SBA-15 was mixed

with 1.5 mL of THF-d8 and stirred for 2 h at room temperature.

The dark green supernatant was then transferred into a NMR

tube and the solid phase was washed with THF and dried in

vacuo. According to the elemental analysis, 76% of the Ru was

removed. In the supernatant, the presence of compound 3 was

demonstrated by comparing 1H and 13C NMR spectra of the

supernatant and corresponding spectra of a fresh solution of 3 in

Beilstein J. Org. Chem. 2011, 7, 22–28.

26

THF-d8. The catalytic activity of the solid phase was then tested

in the RCM of DEDAM in cyclohexane (molar ratio DEDAM/

Ru = 600, T = 30 °C, c0DEDAM = 0.2 mol/L). After 1 h, 55%

conversion of DEDAM was achieved and 62% after 6 h. Only

the RCM products were observed. This indicates that the

remaining Ru species after washing was catalytically active;

however, these species were rapidly deactivated.

Figure 7 shows high-resolution spectra of the Ru 3d–C 1s

photoelectrons of neat compound 3, catalyst 3/SBA-15 and the

same catalyst after leaching in THF. The measured binding

energy of the Ru 3d5/2 electrons (281.2 ± 0.2 eV) was identical

for all these samples and was in accord with the value

280.95 eV reported in literature [25] for a similar compound.

This result indicates that the structure of the Ru complex is not

substantially changed by the immobilization on the support

surface. This conclusion is corroborated by the results of quanti-

tative analysis. For this catalyst, the atomic concentration ratios

Ru/Si = 3.5 × 10−3 and Cl/Ru = 2.0 were obtained from inte-

grated intensities of Ru 3d, Si 2p and Cl 2p photoemission lines.

For the sample leached by THF, the ratio Ru/Si = 9 × 10−4,

which is in accord with the amount of Ru removed from the

support by leaching as determined from elemental analysis.

Figure 7: Spectra of the Ru 3d–C 1s photoelectrons for neat com-pound 3 (1), catalyst sample 3/SBA-15 (2) and catalyst sample 3/SBA-15 after leaching in THF (3).

The results obtained suggest that the Ru species in 3/SBA-15

were attached to the sieves by a non-covalent interaction (prob-

ably via physical adsorption). The small residual Ru content,

which was not possible to remove from 3/SBA-15 with THF at

room temperature, might be due to 3 more firmly attached to the

surface (e.g., adsorbed at special sites on the surface, or

embedded into micropores, etc.). Because of the low concentra-

tion of these species, we have not been able to investigate their

bonding to the surface in greater detail.

ConclusionThe Hoveyda–Grubbs type catalyst 3 was immobilized on

mesoporous molecular sieves MCM-41 and SBA-15 by mixing

a suspension of 3 and sieves in toluene at room temperature.

The immobilization proceeded quickly and almost quantita-

tively. Heterogeneous catalysts prepared in this way exhibited

high activity and 100% selectivity in the RCM of 1,7-octadiene

and diethyl diallylmalonate, in the metathesis of methyl oleate

and methyl 10-undecenoate, and in the ROMP of cyclooctene.

Ru leaching was found to depend on the polarity of substrate

and solvent used. The lowest leaching was found for the RCM

of 1,7-octadiene in cyclohexane – 0.04% of catalyst Ru content.

On the other hand, for the RCM of diethyl diallylmalonate in di-

chloromethane, leaching reached 14% of initial Ru content in

the catalysts.

Results from UV–vis and XPS studies suggested that 3 was at-

tached to the sieve surface by non-covalent interactions.

Approximately 76% of the Ru content could be recovered from

the sieve as 3 (as shown by NMR) by washing with THF at

room temperature (indicating physical adsorption of 3 on the

sieve). The residual Ru species on the sieve exhibited catalytic

activity in RCM but were rapidly deactivated.

The advantage of these catalysts is their simple preparation,

avoiding support modification with special linker molecules. In

nonpolar systems, they can function as true efficient hetero-

geneous catalysts. In the case of polar systems, the possibility of

Ru leaching to a significant extent should be taken into account.

ExperimentalMaterialsToluene was dried overnight over anhydrous Na2SO4, then

distilled with Na and stored over molecular sieves 4 Å. Benzene

(Lach-Ner, Czech Republic) was dried over anhydrous Na2SO4,

distilled with P2O5 and then with NaH in vacuo. Dichloro-

methane (Lach-Ner) was dried overnight over anhydrous CaCl2,

then distilled with P2O5 and stored over molecular sieves 4 Å.

Cyclohexane was distilled with P2O5, then dried with CaH2 and

stored over molecular sieves 4 Å. Diethyl diallylmalonate

(Sigma-Aldrich, purity 98%), cyclooctene (Janssen Chimica,

purity 95%), 1,7-octadiene (Fluka, purity ≥ 97.0%), methyl

oleate (Research Institute of Inorganic Chemistry, a.s., Czech

Republic, purity 94%: methyl palmitate, methyl stearate and

Beilstein J. Org. Chem. 2011, 7, 22–28.

27

methyl linolate as the main impurities) were used as received.

Methyl 10-undecenoate was prepared from 10-undecenoic acid

[26]. The Ru complex 3 was purchased from Zannan Pharma.

Ltd., China.

The synthesis of siliceous MCM-41 and SBA-15 was performed

according to the procedure described in [27]. Their textural

characteristics were determined for MCM-41 and SBA-15, res-

pectively, from nitrogen adsorption isotherms: surface area

SBET = 972 and 934 m²/g, average pore diameter d = 3.8 and 6.9

nm and volume of pores V = 1.14 and 0.96 cm³/g. The particle

size (by SEM) ranged from 1 to 3 μm for all supports used.

TechniquesUV–vis spectra of catalysts were recorded with a Perkin-Elmer

Lambda 950 spectrometer. A Spectralon integration sphere was

applied to collect diffuse reflectance spectra of powder samples.

Spectralon served also as a reference. Catalyst samples were

placed in a quartz cuvette under an Ar atmosphere. 1H

(300 MHz) and 13C (75 MHz) NMR spectra were recorded on a

Varian Mercury 300 spectrometer in THF-d8 at 25 °C.

The photoelectron spectra of the samples were measured with

an ESCA 310 (Scienta, Sweden) spectrometer equipped with a

hemispherical electron analyzer operated in a fixed transmis-

sion mode. Monochromatic Al Kα radiation was used for the

electron excitation. The samples were spread on aluminium

plates and the spectra were recorded at room temperature. The

Si 2p, O 1s, Cl 2p, C 1s and Ru 3d photoelectrons were

measured. Sample charging was corrected using the C 1s peak

at 284.8 eV as internal standard. For the overlapping C 1s and

Ru 3d lines, the contributions of individual components were

determined by curve fitting. The spectra were curve-fitted after

subtraction of the Shirley background [28] using a

Gaussian–Lorentzian line shape. The decomposition of the Ru

3d and C 1s profiles was made subject to the constraints of the

constant Ru 3d doublet separation (4.17 eV) and the constant

Ru 3d5/2/Ru 3d3/2 intensity ratio equal to the ratio of the corres-

ponding partial photoionization cross sections (1.45) [29].

Quantification of the elemental concentrations was accom-

plished by correcting photoelectron peak intensities for their

cross sections, analyzer transmission function and assuming a

homogeneous sample.

A high-resolution gas chromatograph Agilent 6890 with DB-5

column (length: 50 m, inner diameter: 320 μm, stationary phase

thickness: 1 μm) was used for reaction product analysis.

Nonane was used as an internal standard when required. The Ru

content was determined by ICP-MS (by Institute of Analytical

Chemistry, ICT, Prague).

Catalyst preparationAbout 1 g of support was transferred into a Schlenk tube and

dried for 3 h at 300 °C in vacuo. After drying, the Schlenk tube

was filled with argon. Then 10–20 mL of toluene and a calcu-

lated amount of 3 was added with stirring at room temperature.

After stirring for 30 min, the solid phase turned green and the

supernatant became colorless. Then, the supernatant was

removed by filtration and the solid catalyst was washed two

times with 10 mL toluene. Finally, the catalyst was dried in

vacuo at room temperature.

Catalytic experimentsCatalytic experiments were performed in Schlenk tubes under

an Ar atmosphere in CH2Cl2 or cyclohexane. In a typical

experiment, 1,7-octadiene (225 mg, 2.05 mmol) was added to 3/

SBA-15 (34 mg, 3.4 μmol of Ru) in cyclohexane (10 mL) at

30 °C with stirring. Samples of the reaction mixture (100 μL)

were taken at given intervals, quenched with ethyl vinyl ether

and analyzed by GC. In the ROMP experiments, the reaction

conditions were similar to those for the RCM of 1,7-octadiene,

with an initial concentration of cyclooctene of 0.6 mol/L. After

3 h, the reaction mixture was terminated with ethyl vinyl ether,

the solid catalyst was separated by centrifugation and the

polymer isolated by precipitation in methanol (containing 2,6-

di-tert-butyl-p-cresol as an antioxidant). The polymer yield was

determined gravimetrically. The molecular weight was deter-

mined by SEC and the values related to the polystyrene stan-

dards.

AcknowledgementsThe authors thank M. Lamač (J. Heyrovský Institute) for

recording NMR spectra and J. Sedláček (Department of Phys-

ical and Macromolecular Chemistry, Charles University in

Prague) for determination of polymer molecular weights by

SEC. Financial support from the Grant Agency of the Academy

of Science of the Czech Republic (project IAA400400805), and

the Academy of Sciences of the Czech Republic (project

KAN100400701) is gratefully acknowledged.

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